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Pumps - Specific Speed

Characterizing of impeller types in pumps in a unique and coherent manner.

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Specific Speed is a number characterizing the type of impeller in pump in a unique and coherent manner. Specific speed is determined independent of the pump size and can be useful when comparing different pump designs.

  • Specific Speed identifies the geometrically similarity of pumps

Specific speed can be expressed as

Ns = n q1/2 / h3/4                              (1)

where

Ns = specific speed

n = pump shaft rotational speed (rpm)

q = flow rate (m3/h, l/s, l/min, m3/min, US gpm, British gpm) at Best Efficiency Point (BEP)

h = head rise (m, ft)

Online Specific Speed Pump Calculator

The calculator below can used to calculate the specific speed:

Note! When comparing pumps and their documentation - be aware of the units used.

Typical values for specific speed - Ns - for different designs in US units (US gpm, ft)

  • radial flow pump design - 500 < Ns < 4000 - typical for centrifugal impeller pumps with radial vanes - double and single suction. Francis vane impellers in the upper range
  • mixed flow pump design - 2000 < Ns < 8000 - more typical for mixed impeller single suction pumps
  • axial flow pump design - 7000 < Ns < 20000 - typical for propellers and axial fans

Convert Specific Speed between US units (gpm) and Metric units (m3/h)

  • Ns(US gpm, ft) = 0.861 Ns(m3/h, m)
  • Ns(British gpm, ft) = 0.787 Ns(m3/h, m)

when

  • 1 US gpm = 0.2271 m3/h
  • 1 British gpm = 0.2728  m3/h
  • 1 ft = 0.3048 m

Example - Specific Speed for a Pump with different units

A pump has the capacity 1500 US gal/min (1249 British gpm, 340 m3/h, 5667 liters/min) at 100 ft(30.5 m) of head and is rotating at 1760 rev/min.

US gpm, ft

Ns(US gpm, ft) = (1760 rev/min) (1500 gal/min)1/2 / (100 ft)3/4

    = 2156

British gpm, ft

Ns(British gpm, ft) = (1760 rev/min) (1249 gal/min)1/2 / (100 ft)3/4

    = 1967

m3/h, m

Ns(m3/h, m) = (1760 rev/min) (340 m3/h)1/2 / (30.5 m)3/4

    = 2500

liter/min, m

Ns(liters/min, m) = (1760 rev/min) (5667 liters/min)1/2 / (30.5 m)3/4

    = 10209

Specific Speed Hydro-power Turbines

Specific speed is also used when evaluating hydro-power turbines. Typical range of turbine specific speeds:

  • Francis turbine : 70 - 500
  • Kaplan or propeller turbine : 100 - 350
  • Pelton, Turgo or Cross-flow turbines : 10 - 80  
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Related Topics

  • Pumps

    Piping systems and pumps - centrifugal pumps, displacement pumps - cavitation, viscosity, head and pressure, power consumption and more.

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